Polyaxial bone screw

ABSTRACT

A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a head, a retaining ring and a nut. The external capture structure surface includes a threaded portion and a smooth portion; the smooth portion for slidably mating with the retaining ring and the threaded portion for rotatable attachment to the nut within a cavity of the head. The nut fixes the retaining ring to the shank and also provides a tool engagement formation for driving the shank body into bone.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/317,156 filed Oct. 11, 2011 which is a continuation of U.S. patentapplication Ser. No. 12/384,498, filed Apr. 6, 2009, now U.S. Pat. No.8,034,087, which is a continuation of U.S. patent application Ser. No.10/980,534, filed Nov. 3, 2004, now U.S. Pat. No. 7,513,905, thedisclosures of all of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention is directed to polyaxial bone screws for use inbone surgery, particularly spinal surgery. Such screws have a head thatcan swivel about a shank of the bone screw, allowing the head to bepositioned in any of a number of angular configurations relative to theshank.

Many spinal surgery procedures require securing various implants to boneand especially to vertebrae along the spine. For example, elongate rodsare often required that extend along the spine to provide support tovertebrae that have been damaged or weakened due to injury, disease orthe like. Such rods are often supported by certain vertebrae and supportother vertebrae. The most common mechanism for joining the rod to thespine is to implant bone screws into certain vertebrae which then inturn support the rod or are supported by the rod. Bone screws of thistype may have a fixed head relative to a shank thereof. In the fixedbone screws, the head cannot be moved relative to the shank and the rodmust be favorably positioned in order for it to be placed within thehead. This is sometimes very difficult or impossible to do.

Polyaxial bone screws allow swivelling of the head relative to the shankto accommodate placement of the rod and are most commonly used. Inparticular, a polyaxial bone screw allows rotation of the head about theshank until a desired rotational position is achieved for the headrelative to the shank, after which the rod can be laid or urged into achannel in the head. The position of the head is eventually locked withrespect to movement relative to the shank by friction produced bycombination of parts or other means before or after the rod is insertedin the head. Often, the rod is retained in the head and/or pressure isexerted against the rod by a closure top, plug or the like to capturethe rod in the head of the screw.

Because such implants are for placement within the human body, it isdesirable for the implant to have as little effect on the body aspossible. Consequently, it is quite desirable for the implants to have arelatively small profile both in height and width. It is also desirablethat the implants be lightweight.

One undesirable attribute of some of the swivel-head implants is theneed for a multitude of components that may loosen or even disassemblewithin the body. It is most undesirable for components to be free tomove around in the body after the completion of surgery. Loosening ofcomponents relative to each other may result in related undesirablemovement of the bone or vertebra that the implant was intended tostabilize. Consequently, it is desirable for there to be a lightweight,low profile polyaxial bone screw which assembles in such a manner thatsubsequent pieces lock preceding pieces within the overall structure, sothat there is less likelihood that the various pieces of the structurewill undesirably disassemble.

SUMMARY OF THE INVENTION

A polyaxial bone screw assembly of the present invention includes ashank having a body for fixation to a bone. Integral with the shank andextending axially upwardly and outwardly therefrom is a capturestructure. The capture structure has a radially projecting outer surfacethat is substantially cylindrical and that further includes a helicallywound structure, such as a thread and an adjacent smooth surfacedportion. The upper end of the shank is convexly curved.

The bone screw assembly further includes a head having a top portion anda base. The top portion is open and has a channel. The base also isupwardly open and includes an inner seating surface partially defining acavity and has a lower aperture or opening. The channel of the topportion communicates with the cavity, which in turn communicates with anexterior of the base of the head through the base opening. The baseopening is sized and shaped to receive the capture structure of theshank into the head cavity.

The bone screw assembly further includes a closed retaining ring thathas an internal surface defining a bore, the internal surface sized andshaped to mate with the smooth surface portion of the capture structurewhen the retaining ring and the capture structure are coaxially alignedwithin the head cavity and the capture structure is received in theretaining ring bore.

The external surface of the retaining ring is configured to be inslidable mating engagement with the surface defining the cavity of thehead. Preferably, the retaining ring external surface and the matinghead inner surface are substantially spherical. However, it is notedthat the mating surfaces may be of another shape, such as conical ortapered, especially for the head cavity inner surface. The cooperatingshapes of the retaining ring external surface and the head inner surfaceenable selective angular positioning of the shank body with respect tothe head.

The retaining ring is fixed to the capture structure with a threadedfastener or nut. The nut includes a faceted outer surface and aninternal surface having a helically wound structure configured formating with the outer helically wound structure of the capturestructure. The nut is top loaded into the head, axially aligned with theshank and attached retaining ring, and then rotated until the retainingring is captured between the nut and the shank body. Preferably, theshank capture structure includes an annular seating surface that abutsagainst the retaining ring at one side thereof when the nut contacts andpresses against the retaining ring at another side thereof.

Also, according to the invention, preferably at least one facet of thenut has a weakened area compressible by an outside pressing forcedirected at the weakened area, such as a set tool. When pressure isplaced on the weakened area, pressure also is placed on the helicallywound structure of the capture structure and the helically woundstructure of the nut. At least one or both of these helically woundstructures are deformed by the pressure that is placed on the weakenedarea of the nut, interlocking the helically wound structures, and thuslocking the retaining ring in a preferred position between the nut andthe shank body.

The nut faceted outer surface also provides a tool engagement formationextending from the capture structure for non-slip engagement by a toolfor driving the shank into bone and may also be cooperatively used forattaching the nut and retaining ring to the capture structure. Whenutilized for driving the shank into bone, a top surface of the retainingring disposed adjacent to the nut faceted outer surface also provides anadditional gripping surface for a driving tool.

Preferably, the head cavity is partially defined by an upper shoulderportion having planar surface portions disposed perpendicular to an axisof rotation of the head. An advantage of the upper shoulder is that theretaining ring may be pressed against the upper shoulder during assemblyof the nut on the capture structure, keeping the retaining ring seatedevenly upon the capture structure during such assembly.

A polyaxial bone screw assembly method according to the inventionincludes inserting a closed retaining ring into a head cavity, insertinga capture structure of a bone screw shank through a shank receivingopening of the head and into a cavity thereof and then into theretaining ring; and attaching the capture structure to the retainingring within the head utilizing a nut. A method according to theinvention further includes driving the shank body into bone by rotatingthe shank body with a tool engaged with the nut. Further assembly stepsaccording to the invention include inserting a rod into the channel; andbiasing the rod against a top of the bone screw shank capture structureby rotatably inserting a closure member structure within or onto amating structure of the head.

OBJECTS AND ADVANTAGES OF THE INVENTION

Therefore, it is an object of the present invention to overcome one ormore of the problems with polyaxial bone screw assemblies describedabove. A further object of the invention is to provide apparatus andmethods directed to a shank that uploads into a cavity in a head of thescrew and that utilizes a retaining structure that is downloaded intothe cavity. Another object of the invention is to provide a lightweight,low profile polyaxial bone screw that assembles in such a manner thatthe components cooperate to guard against disassembly afterimplantation. Another object of the invention is to provide bone screwcomponents that do not require overly complicated fasteners orcomplicated methods of fastening within the bone screw head. Anotherobject of the invention is to provide a polyaxial bone screw withfeatures that present frictional or gripping surfaces for boneimplantation tools and may be readily and securely fastened to eachother as well as to the bone. A further object of the invention is toprovide a polyaxial bone screw that is easily removable from the bone,if desired, as a single unit. Furthermore, it is an object of theinvention to provide apparatus and methods that are easy to use andespecially adapted for the intended use thereof and wherein the toolsare comparatively inexpensive to produce.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention.

The drawings constitute a part of this specification and includeexemplary embodiments of the present invention and illustrate variousobjects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a polyaxial bone screwassembly according to the present invention, including a shank, a head,a retaining ring and a nut.

FIG. 2 is an enlarged cross-sectional view of the head, taken along theline 2-2 of FIG. 1, illustrating the retaining ring being inserted intothe head.

FIG. 3 is an enlarged cross-sectional view of the head similar to FIG.2, shown with a retaining ring of FIG. 1, shown in cross-section,disposed in the head and seated on the shank upper end, and also shownwith a nut of FIG. 1 prior to the nut being rotatably inserted onto theshank upper end.

FIG. 4 is an enlarged side elevational view of the bone screw of FIG. 1,shown assembled.

FIG. 5 is an enlarged front elevational and fragmentary view of the bonescrew of FIG. 4, illustrating a set tool with a tool tip pressingagainst the nut and deforming a thread of the shank.

FIG. 6 an enlarged top plan of the bone screw of FIG. 4.

FIG. 7 is an enlarged, cross-sectional view of the head of FIG. 2, shownassembled with the shank, retaining ring and nut, and illustrating aninstallment tool being used to drive the bone screw shank into thevertebra.

FIG. 8 an enlarged side elevational and partially exploded view of theinstalled bone screw of FIG. 7, showing the head with a rod received inthe U-shaped channel and prior to a closure top being rotatably insertedinto the head.

FIG. 9 is a cross-sectional view of the vertebra, head, rod and closuretop of FIG. 8, showing the shank implanted in the vertebra and with thebone screw assembly in a completely assembled and operationalconfiguration with the closure top installed and the shank locked in anangled orientation with respect to the head.

FIG. 10 is a cross-sectional view similar to FIG. 9, showing a break-offhead being removed from the closure top by a socket head of a torquingtool.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

The reference numeral 1 generally represents a polyaxial bone screwapparatus or assembly according to the present invention, operablyutilized by implantation into a vertebra 2 and in conjunction with alongitudinal member or rod 3 so as to secure the rod 3 in a fixedposition relative to the vertebra 2.

With reference to FIGS. 1 and 8, the bone screw assembly 1 includes ashank 6, a head 7, a retaining structure or ring 8, a fastener or nut 9and a closure structure or top 10. The nut 9 includes an inner raisedhelical rib or thread 12, an external hexagonally faceted surface 13 anda slightly radiused or curved top surface 14. The shank 6 is elongateand has a lower body 15, as illustrated in FIG. 9, ending in a tip (notshown). The shank body 15 has a helically wound bone implantable thread17 extending from near the tip (not shown) to near a top 18 of the body15 and extending radially outward therefrom. During use, the body 15utilizing the thread 17 is implanted into the vertebra 2, as is shown inFIG. 9. The shank 6 has an elongate axis of rotation A. It is noted thatthe reference to the words top and bottom, upper and lower, and thelike, as used herein refers to the alignment shown in the variousdrawings, as well as the normal connotations applied to such devices,and is not intended to restrict positioning of the assembly 1 in actualuse.

Extending axially outward and upward from the shank body 15 is a neckregion 20, substantially hyperboloid in configuration, having a minimumradius smaller than a radius at the top 18 of the body 15. Furtherextending axially and outwardly from the neck 20 is a capture structure21 providing a connective or capture portion of the shank 6. The neckregion 20 provides a space between the capture structure 21 and theshank body 15, operably also spaced from the bone or vertebra 2 foradjoining with the head 7. The capture structure 21 has a radially outercylindrical surface 22 with an external helically wound guide andadvancement structure illustrated as a rib or thread 24. The thread 24is located near an upper end 25 of the shank 6 and is sized and shapedto receive the threaded nut 9. Although a simple raised helical rib orthread 24 is shown in the drawings, it is foreseen that other structuresincluding other types of threads, such as buttress and reverse anglethreads, and non threads, such as helically wound flanges withinterlocking surfaces, may be used in alternative embodiments of thepresent invention.

Also located at the shank upper end 25 is a centrally located, axiallyextending and upwardly directed projection or dome 29 that is centrallyradiused so as to have a first radius. The projection 29 is preferablycurved or dome-shaped as shown in the drawings, for positive engagementwith the rod 3, when the bone screw assembly 1 is assembled, as shown inFIG. 10, and in any alignment of the shank 6 relative to the head 7. Incertain embodiments, the surface 29 is smooth. While not required inaccordance with practice of the invention, the domed surface 29 may bescored or knurled to further increase frictional engagement between thedome 29 and the rod 3. Also as illustrated in FIG. 10, preferably thenut top surface 14 has the same or similar first radius as the dome 29to provide a continuous, positive engagement with the rod 3 at anyalignment of the shank 6 with respect to the head 7.

Disposed between the neck 20 and the threads 24 of the capture structure21 is a smooth cylindrical surface 30 terminating at a lower shoulder31. The shoulder 31 is disposed adjacent to the neck 20 and includes anannular seating surface 32 oriented perpendicular to the axis ofrotation A. The surface 32 extends outwardly radially from thecylindrical surface 30. The shoulder 31 divides the smooth cylindricalsurface 30 from the neck 20 of the shank 6. The cylindrical surface 30has a reduced inner radius relative to a maximum radius of the neck 20adjacent the shoulder 31. The cylindrical surface 30 is sized and shapedto slidingly mate with the retaining ring 8 and centrally position theretaining ring 8 in alignment with the shank axis A and also generallycentrally within the head 7, as will be discussed more fully below.

The head 7 has an outer profile that is substantially cylindrical inshape, as shown in FIG. 1. However, the head 7 is not a solid cylinder,but rather includes inner openings, a cavity and a channel describedmore fully hereafter, and being substantially symmetrical with respectto an axis of rotation B of the head 7. The head 7 includes a base 33integral with a pair of upstanding arms 34 and 35. The arms 34 and 35form a U-shaped channel 38 therebetween, defined in part by a lower seat39 having substantially the same radius as the rod 3 for operably snuglyreceiving the rod 3. Each of the arms 34 and 35 has an interior surface41 that includes a partial, helically wound guide and advancementstructure 42. In the illustrated embodiment, the guide and advancementstructure 42 is a partial helically wound flangeform configured to mateunder rotation with a similar structure on the closure top 10, asdescribed more fully below. However, it is foreseen that the guide andadvancement structure 42 could alternatively be a V-shaped thread, abuttress thread, a reverse angle thread or other thread like ornon-thread like helically wound advancement structure for operablyguiding under rotation and advancing the closure top 10 between the arms34 and 35.

Tool engaging apertures 44 are formed on outer surfaces of the arms 34and 35 for holding the head 7 during assembly and also during theimplantation of the shank body 15 into the vertebra 2. The apertures 44are disposed opposite one another and each include respective upwardlyprojecting, hidden inner recesses 45. A holding tool (not shown) issized and shaped to have structure to mate with and to be received inthe apertures 44 and locked into place by pulling the holding toolslightly axially upward relative to the base 33 and toward the channel38 formed by the arms 34 and 35 with a portion of the holding toolextending into the recesses 45. The holding tool and respectiveapertures 44 may be configured for a flexible twist or snap on/snap offengagement wherein the holding tool has legs which splay outwardly toposition the tool for engagement in the apertures 44. It is noted thatthe apertures 44 and the cooperating holding tool may be configured tobe of a variety of sizes and locations along any of the surfaces of thearms 34 and 35.

A chamber or cavity 47 is located within the head base 33 that opensupwardly into the U-shaped channel 38. The cavity 47 is defined in partby a partially spherically shaped inner surface 48, at least a portionof which forms a partial internal hemispherical seat 49. The surface orseat 49 is sized and shaped for mating with the retaining ring 8, asdescribed more fully below. The hemispherically shaped surface 49 has asecond radius associated therewith. At the opening into the U-shapedchannel 38, the cavity 47 is defined in part by a discontinuous shoulderor upper coplanar seat 50 disposed on each of the arms 34 and 35extending radially and substantially perpendicular to the axis B, asillustrated in FIG. 2.

At a bottom of the base 33, the cavity 47 communicates with asubstantially circular bore 52 opening to an exterior of the base 33.The bore 52 is coaxial with the rotational axis B of the head 7. Thebore 52 is defined at least in part by a restrictive neck 54 that has aradius that is smaller than an outer radius of the ring 8, as will bediscussed further below, so as to form a restrictive constriction at thelocation of the neck 54 relative to the retaining ring 8 to prevent thering 8 from passing between the cavity 47 and the lower exterior of thebase 33 of the head 7. However, it is foreseen that the retaining ring 8could be compressible and thus loadable through the neck 54 and thenallowed to expand and fully seat in the spherical seating surface 49. Abevel 55 extends between the neck 54 and the bottom exterior of the base33.

The retaining ring 8 is used to retain the capture structure 21 of theshank 6 within the head 7. The retaining ring 8, best illustrated byFIGS. 1-3, has an operational central axis that is the same as theelongate axis A associated with the shank 6, but when the retaining ring8 is separated from the shank 6, the axis of rotation is identified asaxis C, as shown in FIG. 1. The ring 8 has a central bore 57 disposedalong the central axis C, with the central bore 57 passing entirelythrough the retaining ring 8 from a top surface 58 to a bottom surface59 thereof. The bore 57 is sized and shaped so that the ring 8 fitssnugly but slidably over the shank capture structure 21 and outercylindrical surface 30 in such a manner as to allow sliding axialmovement therebetween under certain conditions, as described below. Asurface 60 defining the bore 57 is smooth and has a radius configured tobe only slightly larger than an outer radius of the cylindrical surface30, providing for slidable mating engagement between the surface 60 andthe surface 30. As will be described subsequently in more detail, theshank capture structure 21 is uploadable into the head 7, and throughthe ring 8 that is already disposed in the head 7, by axially slidingthe capture structure 21 through the ring central bore 57 until the ringbottom surface 59 is seated on the annular surface 32 of the shank 6, asillustrated in FIG. 3.

To secure the retaining ring 8 within the head 7, the inner thread 12 ofthe nut 9 is mated to the outer thread 24 of the capture structure 21.Similar to the thread 24, although a simple raised helical rib or thread12 is shown in the drawings, it is foreseen that other structuresincluding other types of threads, such as buttress and reverse anglethreads, and non threads, such as helically wound flanges withinterlocking surfaces, may be used in alternative embodiments of thepresent invention.

The mating of the nut inner thread 12 and the capture end outer thread24 is aided by the shoulder 50 of the head 7. As illustrated in FIG. 3,after receiving the retaining ring 8 thereon, the shank 6 may be movedupwardly, until the top surface 58 of the ring 8 abuts the flat shoulder50 at each of the arms 34 and 35, providing a relatively stable positionfor receiving the nut 9. The nut 9 is then top loaded into the head 7through the channel 38, placed in axial alignment with the shank 6,lowered onto the shank capture structure 21, and rotated in a clock-wisedirection when viewed from above. The nut 9 may be installed with asocket-type tool, similar to the tool 90 shown in FIG. 7 utilized forimplanting the assembly 1 in a vertebra 2. The socket-type tool mateswith the external faceted hexagonal surface 13, and is rotated anddriven downward until the bottom surface 59 of the ring 8 abuts theannular surface 32 of the lower shoulder 31 and is frictionally fixedthereto. When the ring 8 abuts the annular surface 32, the dome 29protrudes axially above the nut 9 with the nut top surface 14 disposedcontiguous to the dome 29. The dome 29 and the top surface 14 preferablyforming a continuous curved perimeter, the surface 14 extending thefirst radius of the dome 29, as illustrated in FIG. 4.

To further ensure frictional engagement between the nut 9 and thecapture structure 21 of the shank 6, the nut 9 includes one or moreweakened areas 61 located along the faceted surface 13 thereof. As shownin FIG. 5, a set tool 62 having a tip 63 passes between the upstandingarms 34 and 35 of the head 7 and pushes against the nut 9 at theweakened area 61, the tip 63 indenting the area 61, forming anindentation or deformation D on the nut surface, and also pressingagainst the thread 12 and/or the thread 24, creating or causing adeformed thread portion or portions 64, interlocking the threads 12 and24, which in turn lodges the ring 8 in a fixed position with respect tothe shank 6. The deformed thread portion or portions 64 preventcounter-clockwise rotation of the nut 9 with respect to the shankcapture structure 21, and thus prevents the nut 9 and the ring 8 frommigrating up and off the shank upper end 25 and into the channel 38,away from the desired position within the head 7.

The ring 8 has a radially outer partially hemispherically shaped surface65 sized and shaped to slidingly mate with the partially hemisphericallyshaped seating surface 49. The surface 65 has a third radiusapproximately equal to the second radius associated with the seatingsurface 49. The third radius of the ring surface 65 is substantiallylarger than the first radius associated with the dome 29 and alsosubstantially larger than an inner radius of the neck 54. Although notrequired, it is foreseen that the outer partially spherically shapedsurface 65 may be a high friction surface such as a knurled surface orthe like.

Preferably, the retaining ring 8 is constructed of a metal or othermaterial having sufficient resilience and elasticity so as to allow thering 8 to radially expand slightly outward by downward pressure of thenut 9 on the top surface 58 and under pressure from structure above, aswill be discussed further below. This produces a slight outward radialexpansion in the ring 8 at the shoulder 31 of the shank 6.

The longitudinal member or elongate rod 3 can be any of many differenttypes of implants utilized in reconstructive spinal surgery and thelike, but is normally a cylindrical elongate structure having a smooth,cylindrical surface 66 of uniform diameter. The rod 3 is preferablysized and shaped to snugly seat near the bottom of the U-shaped channel38 at the lower seat 39 and, during normal operation, will be positionedslightly above a bottom of the channel 38. In particular, the rod 3normally engages the shank dome 29, as illustrated in FIGS. 8-10, andurges against the dome 29 and, consequently, downwardly against theshank 6 when the assembly 1 is fully assembled. For this to occur, theshank domed surface 29 must extend at least slightly into the space ofthe channel 38 when the retaining ring 8 is snugly seated in the lowerseat 49 of the head cavity 47. The shank 6 and retaining ring 8 arethereby locked or held in position relative to the head 7 by the rod 3firmly pushing downward on the shank domed surface 29. At certaindegrees of inclination of the shank 6 with respect to the head 7, therod 3 may push downward on both the domed surface 29 and a portion ofthe nut top surface 14.

With reference to FIGS. 8, 9 and 10, the closure structure or closuretop 10 can be any of a variety of different types of closure structuresfor use in conjunction with the present invention with suitable matingstructure on the inner or outer surfaces of the upstanding arms 34 and35. The closure top 10 is rotatably received between the spaced arms 34and 35.

The illustrated closure top 10 has a generally cylindrical shaped base67 with an upwardly extending break-off head 68. The base 67 includes ahelically wound guide and advancement structure 71 that is sized, shapedand positioned so as to engage the guide and advancement structure 42 onthe arms 34 and 35 to provide for rotating advancement of the closurestructure 10 into the head 7 when rotated clockwise and, in particular,to cover the top or upwardly open portion of the U-shaped channel 38 tocapture the rod 3, preferably without splaying of the arms 34 and 35.The base 67 further includes a lower point or projection 72. Theprojection 72 provides for increased friction against the rod 3. Theclosure structure 10 operably biases against the rod 3 at and near theprojection or point 72 by advancement and applies pressure to the rod 3under torquing, so that the rod 3 is urged downwardly against the shankdomed top surface 29 that extends into the channel 38. Downward biasingof the shank top surface 29 operably produces a frictional engagementbetween the rod 3 and the surface 29 and also urges the retaining ring 8toward the base 33 of the head 7, so as to frictionally seat theretaining ring spherical surface 65 fixedly against the partial internalspherical seating surface 49 of the head 7, also fixing the shank 6 andretaining ring 8 in a selected, rigid angular position relative to thehead 7.

The closure structure break-off head 68 is secured to the base 67 at aneck 73 that is sized and shaped so as to break away at a preselectedtorque that is designed to properly seat the retaining ring 8 in thehead 7. The break-off head 68 includes an external faceted surface 75, acentral bore 77 and a pass-through slot 78 for receiving manipulatingtools (not shown) and may also include grooves, or other apertures (notshown) for such purpose. The faceted surface 75 is sized and shaped toreceive a conventional mating socket type head 87 of a driving/torquingtool 88 to rotate, drive and torque the closure structure 10.

The closure structure 10 also includes removal tool engagement structurewhich in the present embodiment is in the form of a hex-shaped andaxially aligned aperture 81 disposed in the base 67, as shown in FIGS. 9and 10. The hex aperture 81 is accessible after the break-off head 68breaks away from the base 67. The aperture 81 is coaxial with thehelically wound guide and advancement structure 71 and is designed toreceive a hex tool, of an Allen wrench type, into the aperture 81 forrotating the closure structure base 67 subsequent to installation so asto provide for removal thereof, if necessary. Although a hex-shapedaperture 81 is shown in the drawings, the tool engagement structure maytake a variety of tool-engaging forms and may include one or moreapertures of various shapes, such as a pair of spaced apertures, aleft-hand threaded bore, an easyout engageable step down bore, ahexalobular aperture (as sold under the TORX trademark), or othermulti-lobular aperture or the like.

As shown in phantom in FIG. 2, prior to the assembly 1 being placed inuse according to the invention, the retaining ring 8 is typically firstinserted or top-loaded, into the head U-shaped channel 38, and then intothe cavity 47 to dispose the retaining ring 8 within the inner surface48 of the head 7. As shown in FIG. 2, the retaining ring outer edgedefined by the top surface 58 slides along the inner surface 48 untilthe top surface 58 clears the shoulder 50. Then, the retaining ring 8 isrotated so as to be coaxial with the head 7 (the axis C aligned with theaxis B), the top surface 58 facing the channel 38, and the surface 65seated upon and in sliding engagement with the seating surface 49 asshown in solid lines in FIG. 2.

With reference to FIG. 3, the shank upper end 25 is then inserted orbottom-loaded into the head 7 through the bore 52 defined by the neck54. The retaining ring 8, now disposed in the head 7 is coaxiallyaligned with the shank capture structure 21 at the upper end 25, so thatthe dome 29 passes through the bore 57 and the ring inner surface 60 isslidingly mated to the cylindrical surface 30 of the capture structure21.

As shown in FIG. 3, the retaining ring 8 is preferably pushed upwardlyinto abutment with the shoulder 50 of the head 7 to provide ease ininstallment of the nut 9. The nut 9 is then downloaded through thechannel 38 of the head 7, also as shown in FIG. 3, and then rotatinglymated with the helical thread 24 on the capture structure 21 of theshank 6, until the nut 9 abuts against the top surface 58 of theretaining ring 8. The assembly 1 is now assembled as shown in FIG. 4.

With reference to FIG. 5, the position of the nut 9 on the shank 6 isthen fixed by inserting the set tool 62 between the upstanding arms 34and 35 of the head 7 and pushing against the nut 9 with the set tool tip63 at the weakened area 61, the tip 63 indenting the area 61 and alsopressing against the threads 12 and 24, creating a deformed threadportion or area 64, locking the nut 9 to the capture structure 21, whichin turn lodges the ring 8 in a fixed position with respect to the shank6. The deformed thread portion 64 prevents counter-clockwise rotation ofthe nut 9 with respect to the capture structure 21, and thus preventsthe nut 9 and the ring 8 from migrating up and off the shank upper end25.

At this time the shank 6 is in slidable and rotatable engagement withthe head 7, while the capture structure 21, the nut 9 and the retainingring 8 cooperate to maintain the shank body 15 in rotational relationwith the head 7. According to the embodiment of the invention shown inFIGS. 1-10, only the retaining ring 8 is in slidable engagement with thehead spherical seating surface 49. Both the capture structure 21 and thethreaded portion of the shank body 15 are in spaced relation with thehead 7. The shank body 15 can be rotated through a substantial angularrotation relative to the head 7, both from side to side and from frontto rear so as to substantially provide a universal or ball joint whereinthe angle of rotation is only restricted by engagement of the neck 20 ofthe shank 6 with the neck 54 defining the bore 52 of the head. Anexample of such rotation is shown in FIGS. 9 and 10. The bevel 55provides for a slight increase in the extent of angular rotation of theshank body 15 with respect to the head 7.

With reference to FIG. 7, the assembly 1 is then typically screwed intoa bone, such as the vertebra 2 by rotation of the shank 6 using asuitable driving tool 90 that operably drives and rotates the shank 6 byengagement thereof with the faceted surfaces 13 of the nut 9 and alsofrictional engagement with the top surface 58 of the retaining ring 8.

The rod 3 is eventually positioned within the head U-shaped channel 38,as illustrated in FIG. 8, and the closure top 10 is then inserted intoand advanced between the arms 34 and 35 so as to bias or push againstthe rod 3, the point 72 abrading and piercing the rod 3 as shown in FIG.9.

The break-off head 68 of the closure top 10 is torqued utilizing thedriving/torquing tool 88 disposed about the external faceted surface 75,to a preselected torque, for example 90 to 130 inch pounds, to furtherurge the rod 3 downwardly. The shank dome 29, because it is radiused andsized to extend upwardly into the U-shaped channel 38, is engaged by therod 3 and pushed downwardly when the closure top 10 pushes downwardly onthe rod 3, also as shown in FIG. 9. The downward pressure on the shank 6in turn urges the retaining ring 8 downwardly which causes both adownward and outward thrust of the retaining ring 8.

FIG. 10 illustrates the polyaxial bone screw assembly 1, the rod 3 andthe closure top base 67 positioned in the vertebra 215 with thebreak-off head 68 being removed by the torquing tool 88. The axis A ofthe bone shank 15 is illustrated as not being coaxial with the axis B ofthe head 7 and the shank body 15 is fixed in this angular lockedconfiguration. Other angular configurations can be achieved, as requiredduring installation surgery due to positioning of the rod 3 or the like.

If removal of the assembly 1 from the vertebra 2 is necessary, theassembly 1 can be disassembled by mating an allen-wrench tool (notshown) with the closure top bore 81 to rotate the base 67 in acounter-clockwise direction between and out of the arms 34 and 35. Thedriving tool 90 may then be inserted on the nut 9 and rotated in acounter-clockwise direction. Because of the deformation area 64 of thethreads 12 and/or 24, the nut 9 does not disassemble from the shank 6when the nut is rotated counter-clockwise. Thus, the counter-clockwiserotation of the nut 9 also rotates the shank body 15 along and out ofthe vertebra 2, removing the assembly 1 as a single unit out of thevertebra 2.

It is to be understood that while certain forms of the present inventionhave been illustrated and described herein, it is not to be limited tothe specific forms or arrangement of parts described and shown.

What is claimed and desired to be secured by Letters Patent is asfollows:
 1. A polyaxial bone screw assembly comprising: (a) a shankhaving a body for fixation to a bone and a capture structure extendingfrom the body, the capture structure having an outer surface with afirst helically wound guide and advancement structure and a smoothsurface disposed between the first guide and advancement structure andthe body; (b) a head having a top portion and a base, the head topportion defining an open channel, the base having a seating surfacepartially defining a cavity, the channel communicating with the cavity,the cavity communicating with an exterior of the base through an openingsized and shaped to receive the capture structure therethrough; and (c)a retaining ring having an external surface and an internal surfacedefining a central bore configured to slidably mate with the smoothsurface of the capture structure, the retaining ring external surfaceconfigured to be in slidable mating engagement with the seating surfaceof the head so as to enable selective angular positioning of the shankbody with respect to the head; and (d) a fastener having an internalsurface with a second helically wound guide and advancement structurethereon, the first helically wound guide and advancement structureconfigured to rotatably mate with the second helically wound guide andadvancement structure to secure the fastener and the retaining ring tothe capture structure within the head cavity.
 2. The assembly of claim 1wherein the fastener is a nut having a faceted outer surface with atleast one facet of the nut having a weakened area compressible by anoutside pressing force directed at the weakened area, and when attachedto the capture structure at least one of the first and second guide andadvancement structures being deformable by the outside force pressing onthe weakened area, interlocking the first and second guide andadvancement structures.
 3. The assembly of claim 1 wherein the shankcapture structure includes an annular seating surface and the retainingring has a top and a bottom, the retaining ring bottom abutting theannular seating surface when the retaining ring is received on thecapture structure and the fastener is rotated onto the capture structurewith the first and second guide and advancement structures mated and thefastener pressing against the retaining ring top.
 4. The assembly ofclaim 1 wherein the head cavity is partially defined by an uppershoulder portion having a planar surface disposed perpendicular to anaxis of rotation of the head.
 5. The assembly of claim 4 wherein theupper shoulder portion includes first and second coplanar surfaces. 6.The assembly of claim 1 wherein: (a) the head seating surface that islocated to be in slidable mating engagement with the retaining ringexternal surface is substantially spherical; and (b) the retaining ringexternal surface that is in slidable mating engagement with the headseating surface is substantially spherical.
 7. The assembly of claim 1wherein the retaining ring has a planar top surface adjacent thefastener and the fastener is a nut having a faceted outer surface, theplanar top surface and faceted outer surface cooperating to providenon-slip engagement by a tool for driving the shank body into bone. 8.The assembly of claim 1 wherein the first helically wound guide andadvancement structure is a raised helical rib.
 9. The assembly of claim1 wherein the second helically wound guide and advancement structure isa raised helical rib.
 10. The assembly of claim 1 wherein the retainingring is sized and shaped to be loadable into the head through the openchannel and the shank is sized and shape to be loadable into the headthrough the base opening.
 11. The assembly of claim 1 further comprisinga closure structure insertable into the head, the closure structure foroperably urging the shank in a direction to frictionally lock theposition of the retaining ring external surface relative to the headseating surface, thereby locking the shank body in a selected angle withrespect to the head.
 12. The assembly of claim 11 wherein: (a) the headhas upstanding spaced arms defining the open channel, the arms havingguide and advancement structures on an inside surface thereof; and (b)the closure structure is sized and shaped to be positionable between thearms for closing the channel, the closure structure having a closureguide and advancement structure for rotatably mating with the guide andadvancement structures on the arms, biasing the closure structure uponadvancement rotation against a rod disposed in the channel.
 13. Theassembly of claim 11 wherein the capture structure end has a dome sizedand shaped to extend into the channel for engagement with a rod whenreceived in the head and wherein the closure structure is adapted tooperably urge the rod against the dome upon the closure structure beingpositioned in the head.
 14. In a polyaxial bone screw assembly forsurgical implantation and including a shank having an upper end and athreaded body for inserting into a bone and a head having an outwardopening channel adapted to receive a rod within the channel, the headhaving a shank receiving opening, the improvement comprising: (a) acapture structure disposed on the shank upper end sized and configuredto be uploaded through the shank receiving opening, the capturestructure having an outer surface with a first helically wound guide andadvancement structure and a smooth cylindrical surface disposed adjacentthe first helically wound advancement structure; (b) a retaining ringhaving an external surface and a smooth inner cylindrical surfacedefining a bore, the smooth inner cylindrical surface configured toslidably mate with the smooth cylindrical surface of the capturestructure; and (c) a nut having a faceted external surface and aninternal surface with a second helically wound guide and advancementstructure, the first helically wound guide and advancement structureconfigured to rotatably mate with the second helically wound guide andadvancement structure to secure the retaining ring to the capturestructure within the head, when assembled and in a non-lockedconfiguration, the retaining ring external surface enabling selectiveangular positioning of the shank with respect to the head.
 15. Theimprovement of claim 14 wherein: (a) the head has an inner substantiallyspherical seating surface partially defining a cavity, the cavitycommunicating with both the channel and the shank receiving opening; and(b) the retaining ring external surface is substantially spherical andin slidable mating engagement with the head seating surface.
 16. Theimprovement of claim 14 wherein at least one facet of the nut has aweakened area compressible by an outside pressing force directed at theweakened area, and when attached to the capture structure at least oneof the first and second guide and advancement structures beingdeformable by the outside force pressing on the weakened area,interlocking the first and second guide and advancement structures. 17.The improvement of claim 14 wherein the shank capture structure includesan annular seating surface and the retaining ring has a top and abottom, the retaining ring bottom abutting the annular seating surfacewhen the retaining ring is received on the capture structure and the nutis rotated onto the capture structure with the first and second guideand advancement structures mated and the nut pressing against theretaining ring top.
 18. The improvement of claim 14 wherein the firsthelically wound guide and advancement structure is a raised helical rib.19. The improvement of claim 14 wherein the second helically wound guideand advancement structure is a raised helical rib.
 20. A polyaxial bonescrew assembly comprising: (a) a shank having a body for fixation to abone and a capture structure extending from the body, the capturestructure having an annular seating surface and a cylindrical surfacewith a first helically wound guide and advancement structure and asmooth surface disposed between the first guide and advancementstructure and the annular seating surface; (b) a head having a topportion and a base, the head top portion defining an open channel, thebase having a substantially hemispherical seating surface partiallydefining a cavity, the channel communicating with the cavity, the cavitycommunicating with an exterior of the base through an opening sized andshaped to receive the capture structure therethrough; (c) a retainingring having a substantially hemispherical external surface configured toslidingly mate with the substantially hemispherical seating surface ofthe head, and an internal surface defining a central bore configured toslidably mate with the smooth surface of the capture structure, theretaining ring external surface configured to be in slidable matingengagement with the seating surface of the head so as to enableselective angular positioning of the shank body with respect to thehead; and (d) a nut having a faceted outer surface and an internalsurface having a second helically wound guide and advancement structurethereon, the first helically wound guide and advancement structureconfigured to rotatably mate with the second helically wound guide andadvancement structure to frictionally fix the retaining ring between thenut and the annular seating surface of the capture structure, at leastone facet of the nut having a weakened area compressible by an outsidepressing force directed at the weakened area, at least one of the firstand second guide and advancement structures being deformable by theoutside force, thereby interlocking the first and second guide andadvancement structures and locking the retaining ring between the nutand the annular seating surface within the head.
 21. The assembly ofclaim 20 wherein the head cavity is partially defined by an uppershoulder portion having a planar surface disposed perpendicular to anaxis of rotation of the head.
 22. The assembly of claim 20 wherein theupper shoulder portion includes first and second coplanar surfaces. 23.The assembly of claim 20 wherein the retaining ring has a planar topsurface adjacent the nut faceted outer surface, the planar top surfaceand faceted outer surface cooperating to provide non-slip engagement bya tool for driving the shank body into bone.
 24. The assembly of claim20 further comprising a closure structure insertable into the head, theclosure structure for operably urging the shank in a direction tofrictionally lock the position of the retaining ring external surfacerelative to the head seating surface, thereby locking the shank body ina selected angle with respect to the head.
 25. The assembly of claim 24wherein: (a) the head has upstanding spaced arms defining the openchannel, the arms having guide and advancement structures on an insidesurface thereof; and (b) the closure structure is sized and shaped to bepositionable between the arms for closing the channel, the closurestructure having a closure guide and advancement structure for rotatablymating with the guide and advancement structures on the arms, biasingthe closure structure upon advancement rotation against a rod disposedin the channel.
 26. The assembly of claim 24 wherein the capturestructure end has a dome sized and shaped to extend into the channel forengagement with a rod when received in the head and wherein the closurestructure is adapted to operably urge the rod against the dome upon theclosure structure being positioned in the head.
 27. A polyaxial bonescrew assembly method comprising: (a) inserting a retaining ring througha channel and into a cavity of a head, the head channel openingoutwardly and adapted to receive a rod within the channel, the headcavity being disposed between and communicating with both the channeland a shank receiving opening, the retaining ring having an innercylindrical surface defining a bore; (b) inserting a capture structureof a bone screw shank through the shank receiving opening of the headand into the cavity thereof, the capture structure being integral withan elongate threaded shank body and having an outer surface with a firsthelically wound guide and advancement structure thereon adjacent to asmooth cylindrical surface; and (c) attaching the capture structure tothe retaining ring within the cavity by inserting the capture structurethrough the bore until the inner cylindrical surface of the retainingring is in sliding engagement with the smooth cylindrical surface of thecapture structure, and inserting a nut having a second helically woundguide and advancement structure onto the capture structure and rotatingthe nut, mating the first and second helically wound guide andadvancement structures.
 28. The method of claim 27 further comprising:(d) driving the shank body into bone by rotating the shank body with atool engaged with the nut.
 29. The method of claim 28 furthercomprising: (e) subsequently inserting a rod into the channel; and (f)biasing the rod against the capture structure by inserting a closurestructure into the channel.
 30. A method of assembling a polyaxial bonescrew comprising the steps of: (a) providing a bone screw shank, head,retaining ring and nut; (b) providing the shank with an upper threadedcapture structure; (c) providing the retaining ring with an innercylindrical surface defining a bore; (d) providing the head with acentral cavity and a shank receiving bore connecting the cavity with anunderside of the head; (e) loading the retaining ring into the cavity;(f) uploading the shank capture structure into the cavity through theshank receiving bore and the retaining ring bore; and (g) screwing thenut onto the shank capture structure while within the head.
 31. Themethod according to claim 30 including the steps of: (a) providing thehead with an upwardly open channel that communicates with the cavity;and (b) downloading the retaining ring and the nut through the channelinto the cavity.